1. Field of the invention
The present invention concerns a process for producing a preparation for use in the surface protection of zinc against "white rust" in both solid zinc and zinc-coated articles. The invention encompasses said preparation and a process for the protection, optionally temporary, of zinc against white rust.
2. Description of the prior art
Zinc, of course, is a metal which is widely used particularly in the protection of steels against corrosion (galvanization).
In a relatively low humidity atmosphere, zinc carbonate (the "patina") forms on the zinc surface which renders the metal less sensitive to any further atmospheric corrosion. However, in a humid, acidic or warm atmosphere (as is often the case in industrial or tropical storage) zinc carbonate formation is locally replaced by formation of hydrated zinc hydroxide, generally known as "white rust", which modifies, the appearance of galvanized objects and corrodes them.
The zinc surface must therefore be protected in order to avoid formation of white rust.
The most successful prior art technique uses sodium dichromate solution, Na.sub.2 Cr.sub.2 O.sub.7, which affords suitable protection but whose use is dangerous due to the toxicity of Na.sub.2 Cr.sub.2 O.sub.7.
Other techniques produce reasonable results as far as protection is concerned, but certain drawbacks (such as greasy or oily films, inflammability, etc) limit their use.
The aim of the invention is to overcome these drawbacks by using a preparation based on acrylic and/or methacrylic polymers.
U.S. Pat. No. 2,958,611 has already envisaged treatment of zinc articles with acrylic polymer-based preparations. However, these preparations are certainly not intended to protect zinc against white rust but to form a priming layer to allow a subsequent layer of paint to adhere to it, covering the components and rendering them unsuitable for their currently envisaged purpose.
Thus the acrylic copolymer described in U.S. Pat. No. 2,958,611 is produced in volatile organic solvents or in aqueous dispersion and requires use of a surfactant or wetting agent (Triton X-200).
An exclusively or essentially organic solution can only be applied at relatively low temperatures compared with those at which articles leave galvanization baths.
Use of a surfactant in a dispersion or emulsion renders it unsuitable for weather protection of zinc.